Generally, argon gas is contained in the atmosphere by 0.93%, and therefore, argon gas is obtained as a byproduct in the factory in which nitrogen and oxygen gases are liquified and separated. There is no process for obtaining only argon gas. However, as argon gas is required in greater amounts than nitrogen and oxygen gases, another argon manufacturing source is required. In the ammonia purge gas, argon is contained by 4-6%, and hydrogen is contained by about 60%. Usually, therefore, the ammonia purge gas is burned as a fuel, and the argon gas is released into the atmosphere.
The conventional argon recovery method is carried out in the following manner. The ammonia purge gas is passed through a drier to remove the moisture, and is liquified to recover hydrogen. Then nitrogen is removed, and then, methane or argon is separated. Thus argon is recovered through multiple stage, cryogenic distillation, and therefore, the facility cost and the liquefying energy are required too much, with the result that the method is not widely used.
In BOC of the United States (U.S. Pat. No. 4,689,062, and 4,752,311, and European Patent 0239234), hydrogen is recovered by applying a cryogenic method or membrane separating method. Then the hydrogen depleted gas mixture in which argon is contained by 12% is subjected to a concentration process by applying a PSA (pressure swing adsorption) method until argon is concentrated to 21% (hydrogen: 26.4%, argon: 21.4%, and nitrogen: 52.2%). The resultant mixture gas is sent to a cold box to liquify it, and then, the argon is recovered by using a distillation column which consists of more than 40 trays.